Method of purifying nitrobenzene



Jan. 24, 1939. H. J. M CREARY METHOD OF PURIFYING NITROBENZENE Filed Sept. 20, 1935 7% wh Z r w 7 UM Patented Jan. 24, 1939 UNi'l' GEE-HQ WETHOD F PURIFYING NITROBENZENE Application September 20, 1935, Serial No. 41,366

2 Claims.

My invention relates to a process for purifying dielectrics and specifically to a process for making Kerr cells for television systems.

The invention comprises a process for prepar- 5 ing nitrobenzene or other liquid dielectrics for use in a Kerr cell or wherever it may be applicab le and the method of installing the liquid in the cell and otherwise preparing a cell for obtaining the Kerr effect in a television system or other fields of application.

The Kerr efiect is the electric double refraction obtained in some substances, particularly liquid dielectrics and noted first by Kerr. The Kerr cell when used in proper combination with other electro-optical elements is the best light control device for purposes of television. In a cell of this type an ordinary and extraordinary beam of light acquire a phase difference which is proportional to the square of the applied voltage. A

light valve employing this effect usually employs a prism for polarizing light waves and a second prism called an analyzing prism by means of which the outgoing light may be controlled electrically. Nitrobenzene and nitrobenzol are liquids having high values of the Kerr constant.

The average Kerr constant of this liquid is approximately equal to 4X10- Abs. The Kerr constant depends considerably upon temperature and the degree of purity of the fluid is of very great importance.

Impurities in a Kerr cell are disturbing in several respects. First they cause undesirable field disturbances in the Kerr cell. Second, they may reduce the value of the Kerr constant itself.

Third, they cause a high conductivity with the resuit that at a desired voltage the conduction current becomes disturbingly high. Fourth, a lower breakdown strength is also disturbing. Iherefore, impurities may cause electrolytic conduction and a rapid decrease of the Kerr constant, making it more difficult to build up a proper potential across the plates and the material less eifective when the potential has been built up. A lag in the Kerr cell action is introduced from impurities because of the electrical leakage thus introduced and the time required to electrically polarize the cell.

For these reasons the liquid to be used must be subjected to a special process involving purification, distillation and a vacuum sealing 01f process.

My invention relates to a method which has been found to yield the best possible results in satisfying the requirements to be met in constructing 2. Kerr cell for television purposes. To

my knowledge no process has been developed previous to my invention which will yield wholly satisfactory results, although many attempts have heretofore been made to obtain absolutely pure nitrobenzene. 5 It is accordingly an object of my invention to provide a process for obtaining a high purification of the liquid dielectric known as nitrobenzene C6H5NO2 and trinitrobenzene CsH3(NO2) 3. 10

It is another object of my invention to provide a means of constructing Kerr cells, which consists of a continuous process of purification including distillation, evacuation of the cells, selective filling of a plurality of cells, electrical 15 polarization of the cells to reduce gassing, and selective sealing off of a plurality of such cells, all of these functions being performed in a continuous process.

It is a further object of my invention to construct Kerr cells by means of which the best possible results are obtained in a television system.

Other equally important objects will become apparent from a perusal of the application comprising the specification, which sets forth the means and the steps of the process employed in the invention, and from the claims which particularly point out the novel features of the invention. The means for carrying out the process b constituting the invention is illustrated in the accompanying drawing which illustrates the apparatus used in carrying out the present process.

My invention covers a process of constructing Kerr cells, which is divided into six steps, namely:

1. Treatment of nitrobenzene with sodium carbonate to neutralize such acids as may be left from nitrating.

2. Washing with distilled water. 40

3. Drying with sodium sulphate.

4. Filtering and sealing into vacuum flask.

5. Distilling the nitrobenzene into the Kerr cells or into a second vacuum flask.

6. Redistilling from a second vacuum flask into 45 Kerr cells.

Referring to the drawing, l is a Pyrex glass tube approximately 1" in diameter and 3 ft. long, sealed at one end to a smaller tube 2 which is approximately in diameter, and 3" long. 5 Tubes l and 2 comprise the filtering column. A porcelain straining disc 3 is installed with a loose fit into the tube l and rests on the constricted portion 4 of the glass tube 3. Five discs of filter paper 5 are rammed down on top of the porcelain strainer 3 by means of a glass tube (not shown) slightly smaller than the tube I. The filter paper discs are about larger than the inside bore of tube I, and are pushed into place one at a time. By this process the proper grade of strainer is installed in tube I.

Commercially pure calcium oxide 6 is finely powdered with a mortar and pestle and is then placed on top of the filter paper disc 5. The calcium oxide 6 'is packed down with a glass rod (not shown) so that it forms a column approxi.'

mately six inches deep in the tube I. On top of this column of calcium oxide two discs of filter paper I exactly of the same type as filter paper 5 are rammed into place.

A column approximately 10 long of tightly packed commercially pure anhydrous sodium sulphate 8 is then placed on top of the filter paper discs I. The sodium sulphate has been very finely powdered before using. The remainder of tube I is now filled with the nitrobenzene to be purified and a loose fitting cover 9 is placed ontop of the glass tube i'to keep-dust and other impurities out.

The nitrobenzene which is tozbe purified has first-been treated in accordance with steps I, 2, and 3,'referred to above in'the following manner: About 2000' grams (about 1500 c. c.) of nitrobenzene'having a conductivity of 114x 10- mhos per cubic centimeter and having a resistance of about 87 .8 megohms per cubic centimeter is placed in a separatory funnel and a 4% solution of sodium carbonate (NazCOa) added. The mixture is well shaken by hand, and the nitrobenzene is then decanted into a second .separatory funnel where more sodium carbonate solution is added and the same process repeated. The dirty sodium carbonatesolution in the first funnel is thrown out in the meantime. Then the nitrobenzene is again decanted'from the second separatory funnel back into the first funnel, and'the process described above again repeated. This is continued until the sodium carbonate solution remainscolorless after the shaking process, which usually requires three or four'times.

The procedure followed in step 2 utilizes the same equipmentas step? I and the only difference between the procedure is that distilled water is used instead of the-sodium carbonate, and the process is continued as described in step I three or four times after the decanted water tests neutral to phenolphthalein. Usually 2600 c. c. of distilled water is used;

The preliminary treatment of the nitrobenzene for .the drying process .inxstep 3 consists of placing the nitrobenzene and 200 grams of sodium sulphate in a special flaskwhi'ch is attached to a motor operated agitator; The mixture is agitated for about an hour and is then filtered. A fresh change of 200 grams of sodiumsulphate is then again placed'in the filtered nitrobenzene and agitated-for about one hour; after which the mixture is again filtered. This process is repeated a third time, after which-the nitrobenzene is ready to'be poured into thexfiltering column I, in the manner above described.

The filtering column I-is connected by means of a small tube 2 to the glass bulb I I, which in. turn is connected by a tube I2 to a residual container I3 and by a tube I I to a special flask I5 through a seal-01f constriction I6. The glass bulb II is also connected by tube Ill to an oil vacuum pump (not shown). The flask I5 is special in that it has in its neck i! a capillary I8 so that it can be opened in a vacuum in a manner well known in the art of handling rare gasses.

Attention is called to the fact that the filtering column at the top has less vacuum than at the bottom, so that as the liquid descends it is subjected to a progressively increasing vacuum.

The entire glass structure comprising filtration column I, bulb Ii, residual container I3, and special flask I5, are all .welded into a continuous vacuum-tight envelope and mounted with clamps on a tilting structure pivoted so that the envelope may be tilted so that either tube I2 or tube I4 may be filled with processed nitrobenzene from the tube 2 of the filtering column.

The glass structure of Fig. l is tilted on its pivot sothat the aperture .of tube 2 is directly over the aperture oftube I2. pump connected to tube It! with a rubber'hose is started and the nitrobenzene which has been poured'into the top of filtration columnxI willseep downthe filtration column until it drips from the apertureof tube v2 into tube I2 andinto'theresidual container I3.

After a few teaspoonsful have dripped into .the container 83, the glass structure is tilted about its pivot in such a manner. that the nitrobenzene from tube 2 will drip into tube I4 and run down into'the special flask I5; The topof the filtration column is always kept full of nitrobenzeneand is never-allowed to get downto the top' surface of the sodium sulphate. When the special flask lihas been filled to within a short distance of the seal-off constriction I6, the glass structure is tilted on its pivot ,until the nitrobenzene again drips into tube I2; is 'then warmed to vaporize the nitrobenzene on the inner surface and the special flask I5 is then sealed oft.

My invention .is not'limitedtto the particular arrangement of apparatus as shown in the-drawing in carrying out the process, but may be variously modified without departing'fromthe spirit and scope ofthe invention set forth inthe claims which follow, and I, therefore, claim my invention as broadly as the state of the art permits.

[claim:

1. The method of purifying nitrobenzene, including the steps of forcing the nitrobenzene througha long column of anhydrous sodium sulphate, and subsequently through a long column of calcium'oxide to neutralize any remaining acid radicaL-said columns being maintained under a progressively increasing vacuum.

2. The method of purifying and discoloring nitrobenzene, including the steps of forcing the nitrobenzene through a long column of anhydrous sodium sulphate; and subsequently through a long column of calciumoxide to neutralize any remaining acid radical, said neutralizing column being maintained under a progressively increasing vacuum.

HAROLD J. McCREARY.

The oil vacuum' The seal-off constriction I5 2 

